The present invention relates generally to microwave circuits and, more particularly, to a dielectric resonator filter useful at L-band, the filter being structured so as to be relatively insensitive to vibration while retaining a relatively high loaded Q.
Resonators are important components in microwave radar and communication circuits. They create, filter and select frequencies in oscillators, amplifiers and tuners. Fields inside a resonator store energy at the resonant frequency where equal storage of electric and magnetic energies occurs.
The figure of merit for assessing the performance or quality of a resonator is the quality factor Q. The Q factor is a measure of energy loss or dissipation per cycle as compared to the electromagnetic energy stored in the fields inside the resonator. The Q factor of microwave devices may typically be as high as 10,000. Because the bandwidth of a resonator is inversely proportional to its Q factor, a high Q factor resonator has a narrow bandwidth.
When a resonant circuit or cavity is loaded by a microwave circuit, several different Q factors may be considered. The unloaded Q factor accounts for internal losses in the cavity or resonator itself. For cavity resonators, power loss by conductors, dielectric fills and radiation may contribute to the unloaded Q.
In order to be useful, a cavity or resonator must deliver power to an external load. The power loss due to the presence of an external load in a cavity system results in the external Q factor, which is proportional to the ratio of the energy stored inside the cavity to the external power loss drained from the internal energy reserves. The loaded quality factor is the total Q for the system including power losses both internal and external to the resonator system.
An important resonator circuit at microwave frequencies is the metal cylindrical hollow waveguide resonator. Very high Q factors and the concomitant narrow bandwidths may be achieved with this component. External circuits are coupled to the cavity through transmission line probes. The hollow cylindrical waveguide resonator has many resonance frequencies and accompanying field distributions. Electromagnetic fields can be sustained within a lossless cavity only at a resonant frequency. The resonant cavities are particularly useful as filters.
In recent years there have been increasing requirements on the performance of radar systems which have placed demands on the noise and spurious products performance of the microwave sources. The recent emphasis has been directed toward developing stable oscillators that operate at or close to the radar operating frequency. This must be done due to the noise enhancement of a source, because of multiplication to a higher frequency, and the spurious products and noise encountered by mixing to a higher frequency.
The present invention relates to a high Q dielectric resonator filter which is at the core of a low noise dielectric resonator oscillator which is phase locked to a low noise, frequency multiplied crystal source. Typically, a dielectric resonator filter includes a resonator fabricated of a material of high dielectric constant to load a cavity, thereby reducing the size of the cavity and, additionally, stabilizing the cavity, since 90 percent of the dielectric field and 65 percent of the magnetic field exist in the dielectric resonator.
According to the prior art, in order to obtain a high value of loaded Q in a dielectric resonator filter, the dielectric resonator is placed centrally within the cavity. Such positioning typically entails mounting the dielectric resonator on a pedestal fabricated of a low loss, low dielectric material in the center of the cavity. This approach approximates a free space mounting of the dielectric resonator, and the central positioning within the cavity minimizes the electric and magnetic field gradients outside the dielectric resonator, thereby reducing the losses encountered by high circulating currents in the conducting boundaries presented by the cavity.
This prior art approach, however, renders the dielectric resonator very susceptible to vibration. Mounted on a pedestal, the dielectric resonator's motion induced by shock or vibration is magnified as a load at the end of a cantilevered beam. The problem is exaggerated for a larger and more massive dielectric resonator required at L-band frequencies. The motion causes variations in the position of the dielectric resonator with respect to the walls of the cavity, which variations affect the distribution of the electric and magnetic fields in the dielectric resonator filter.
It is the need for an improved dielectric resonator filter, having a high quality factor and low insertion loss, operable at L-band, and being relatively insensitive to vibration, that provided the impetus for the present invention.